Liposomes have been used to deliver a wide variety of therapeutic agents. For example, antitumor agents such as actinomycin (U.S. Pat. No. 3,993,754), anthracyclins (U.S. Pat. No. 4,863,739), and vinca alkaloids (U.S. Pat. No. 4,952,408) have been encapsulated in liposomes. More recently, thermosensitive liposomes containing active agents have been prepared and used to deliver the active agent to specific targets in a subject (U.S. Pat. Nos. 6,200,598 and 6,726,925, and Yatvin et al., Science 204:188 (1979). In use, thermosensitive liposomes are delivered to a subject and a target area in the subject is heated. When the thermosensitive liposome reaches the heated area, it undergoes a gel to liquid phase transition and releases the active agent. The success of this technique requires a liposome with a gel to liquid phase transition temperature within the range of temperatures that are obtainable in the subject.
There remains a need in the art for liposomes formulated to encapsulate a therapeutic agent such as an antitumor agent that can undergo a gel to liquid phase transition at a temperature obtainable in a subject. This need and others are met by the present invention.